1. Field of the Invention
The present invention relates to algae-resistant roofing granules. More particularly, it relates to algae-resistant roofing granules of the type which are artificially colored and contain a slow-release bimetallic algicide as a component of the color coating.
2. Reported Development
Roofing granules, both natural and artificially colored granules, are extensively used in roll roofing and asphalt shingle compositions. The roofing granules are embedded in the asphalt coating on the surface of the asphalt-impregnated felt base material, the granules thus forming a coating that provides an adherent, weather-resistant exterior roofing surface.
In warm and humid climates discoloration of asphalt roofing compositions by the growth of algae is of particular concern. To prevent such discoloration the prior art proposed the use of metallic algicides incorporated into roofing granules that are effective in retarding the biological growth of algae. Illustrative examples of prior art compositions are as follows.
U.S. Pat. No. 3,494,727 discloses a microbe resistant roof wherein the roofing material contains embedded therein visually observable metallic chips, such as copper, lead, zinc and iron.
U.S. Pat. No. 3,507,676 discloses zinc-containing coating for roofing granules in the form of zinc, ZnO or ZnS.
U.S. Pat. No. 3,528,842 discloses copper compound-containing algicidal surfacing, the copper compound being cuprous oxide or cuprous bromide.
U.S. Pat. No. 3,598,627 discloses fungus-resistant shingles the outside surface of which are asphalt coated and are covered by a hard, granular weathering material and granular zinc.
U.S. Pat. No. 3,884,706 discloses algicidal roofing granules coated with a substantially water insoluble silicate-clay coating into which small amounts of algicides, e.g. 0.05% to 0.5% zinc algicide and 0.05% to 0.5% copper algicide, are incorporated based on the total weight of the roofing granules.
U.S. Pat. No. 4,092,441 discloses roofing granule treatment by coating the granules with a metallic algicide, such as zinc, copper, nickel and mixtures thereof. The metals are sprayed onto the granules in the form of molten droplets.
U.S. Pat. No. 5,382,475 discloses algicidal granules having a ceramic coating thereon comprising three layers, the first two of which have a copper compound, and the third layer has a pigment, a borate compound and zinc oxide.
While the teaching of the above-mentioned references greatly advanced the state of the art and provided impetus for the building materials industry in creating algicidal roofing granules, the need for creating enhanced, long-term and slow-release algicide-containing granules still exists. The object of the present invention is to provide for that need.
The present invention provides bimetallic algicide-coated granules in which the two metals act synergistically in preventing/retarding the growth of algae.